Water, - - - - - - . - - 20 02S.
Ammonia, ------ - - 5 drops.
The temperature of this solution should not be over 65°F. In summer it may be necessary to cool it with ice. A dish, somewhat larger than the tissue, should be filled to the depth of about one inch with solution. The tissue is now immersed in it, care being taken to avoid air bubbles as much as possible; hut, should they occur, either on the back or front of the tissue, they must be quickly removed by lightly brushing the surface with the camel's hair mop. The usual time for immersion is about three minutes, but this varies greatly according to the temperature of the solution. A good plan is to remove the tissue immediately it becomes limp, placing it surface side downwards on a squeegeeing board—a plain board covered with sheet zinc—and then removing the surplus moisture with a stroke of the squeegee. Should a squeegeeing board not be available, a sheet of plate glass forms an efficient substitute. The tissue is now raised from the board and hung up to dry by means of clips. The tissue does not become sensitive till nearly dry.
The greatest care must be taken that this operation is carried out under suitable conditions, for after success depends very largely upon it. It is advisable to dry the tissue as quickly as possible and in an atmosphere free from all impurities. A properly constructed drying cupboard is very useful for the purpose, although an ordinary room in which the fire has been burn ing during the day and which has been darkened to prevent the entrance of white light, will do just as well, and tissue left there at night will be ready for use the following morning. Care should be taken not to expose the tissue to even the weakest actinic light, because it is more sensitive than silver paper, and, moreover, any light action set up, continues when the tissue is placed in a dark place. The tissue being quite dry, it is stored in a pressure box ready for use.
Negatives for carbon printing should be vigorous and full of gradation. Do not attempt your first print from a flat, thin negative. Each negative must be provided with what is termed a " safe edge." This is made by painting round the edge of the negative with black varnish, or by sticking a strip of black or yellow paper round the negative on the glass side. Commercial lantern slide binders are very convenient for this purpose because they can be affixed in a few moments. Without this safe edge there is great danger of the insoluble gelatine leaving its support during development.
The negative being prepared we have only to place it in a printing frame, with a piece of the sensitized tissue in contact, and expose to day light as in any ordinary printing process. Now as we are printing on a dark colored surface it is evident that we must have some means for calculating the necessary exposure. Instruments for this purpose are known as
Actinometers, and any of the commercial forms will be found quite simple in use after one or two trials. An alternative to their use is to take a nega tive of equal density to that we are printing in carbon, and, placing it in a frame with a small strip of Solio paper, expose the frames simultaneously to daylight. When the strip of Solio paper is printed to the " pretty " stage, i e., lighter than it would be for toning, we may consider our carbon print is finished and ready for development.
If we pause for a moment to think of what has taken place during printing, we shall see that the image of insoluble gelatine lies on the surface of the pigmented film instead of in contact with the paper supporting it. It follows, therefore, that if we place the " tissue " in hot water, the gelatine between the image and the paper will be dissolved and the insoluble film forming the picture will float away and be lost. Of course this difficulty might be overcome by printing through the paper, but this procedure would give rise to other troubles. The grain of the paper, the long time taken in printing—even when the paper is oiled as suggested by Mr. Blanchard— and the loss in definition, are serious obstacles to the success of this method. The difficulty has been met in a simple manner. If the printed tissue is placed in water for a few moments and then brought into contact with a flat surface excluding air, it will adhere during development. We, therefore, prepare wood, paper or other material on which we desire to transfer our image, by coating it with a layer of insoluble gelatine. Single transfer papers, smooth or rough, white or tinted, may be obtained ready for use, but probably the ideas of the photographer lead him to sometimes require something different to the commercial article and he will prefer to prepare his own. For this purpose take : Nelson's No. r Gelatine, r oz.
Water, ------ - - 2 OZS.
and after letting it soak for some hours, dissolve by gentle heat. Add to this solution very gradually Chrome Alum, 20 grains dissolved in 2 055. water.
stirring vigorously all the time.
With a stiff brush or sponge the chromated gelatine is applied to the surface of the wood or paper and allowed to dry. Ordinary papers require two coats, but Whatman and other rough drawing papers should have three. Should Nelson's Gelatine not be procurable, any other make may be used in its place, but if they are harder the alum must be greatly reduced. Should Heinrich's Hard Gelatine be used, about four grains of Alum would be sufficient. A larger quantity might cause coagulation during mixing.